The process of this invention provides a more economical commercial process than previously available methods for the manufacture of trimellitic anhydride (4-carboxyphthalic anhydride) through the catalytic liquid phase oxidation of pseudocumene. (1,2,4-trimethylbenzene) with air in the presence of acetic acid as reaction solvent, the separation and recovery of crystalline trimellitic acid from the oxidation reaction effluent, the thermal dehydration of trimellitic acid to its anhydride, and the separation and recovery of the anhydride from intermediate oxidation by-products and other oxidation by-product impurities by distillation and/or vaporization process steps.
Pseudocumene is oxidized with air mainly to a mixture of di- and tricarboxy acids in the presence of catalysts comprising cobalt and/or manganese oxidation catalysts under liquid phase conditions, using acetic acid as the reaction solvent.
U.S. Pat. No. 4,587,350, incorporated by reference herein, discloses a process for the oxidation of pseudocumene to trimellitic acid by a catalytic oxidation of pseudocumene with air in the presence of acetic acid in an oxidation zone in the liquid phase with catalysts comprising zirconium, cobalt, and manganese and a source of bromine.
By the use of oxygen as oxidant and a combination of cobalt as metal oxidation catalyst and alpha-methylenic ketones as side chain oxidation initiator or promoter, pseudocumene is oxidized mainly to a mixture of 2-methyl terephthalic acid and 4-methyl isophthalic acid in the presence of acetic acid solvent under liquid phase conditions at atmospheric pressure. Catalytic liquid phase oxidation of pseudocumene with air can be accomplished in the presence of acetic acid solvent and catalysis provided by the combination of heavy metal oxidation catalyst and a source of bromine as disclosed and claimed in U.S. Pat. No. 2,833,816. This oxidation method using a combination of heavy metal oxidation catalyst and a source of bromine to provide catalysis describes the production of 92 weight percent trimellitic acid filter cake product in a two hour reaction at 198.degree. C. (about 390.degree. F.). The theoretical yield of trimellitic acid from pseudocumene is 175 weight percent. However, the oxidation method of U.S. Pat. No. 2,833,816 has been developed to produce total trimellitic acid yields in the range of 135 to 161 weight percent or about 77% to about 92% of theory based on the pseudocumene hydrocarbon feed. By total yield of trimellitic acid is meant all of the trimellitic acid in the oxidation reaction effluent.
The highly developed catalytic liquid phase air oxidation of pseudocumene by the method of U.S. Pat. No. 2,833,816 using catalysis provided by the combination of heavy metal oxidation catalysts therein defined with bromine or a source of bromine under liquid phase oxidation conditions produces total trimellitic acid yields of 135 to 161 weight percent based on commercially available pseudocumene. But, there are also coproduced trimesic acid, iso- and terephthalic acids, 4-methylorthophthalic acid, 2-methylterephthalic acid, 4-methylisophthalic acid and formyl phthalic acids in amounts as to present substantial problems in the recovery of high quality trimellitic acid, dehydration of trimellitic acid to its intramolecular anhydride and recovery of that anhydride.
Another problem in the manufacture of trimellitic acid through the oxidation of pseudocumene to trimellitic acid in the presence of acetic acid comes from the relatively high solubility of trimellitic acid in acetic acid. This solubility goes from about 1.0 pound per 100 pounds glacial acetic acid at 80.degree. F. to 6.5 pounds per 100 pounds glacial acetic acid at 220.degree. F. The presence of water in the acetic acid increases the solubility of trimellitic acid so that in aqueous acetic acid solvent having 82 to 85% acetic acid and 18 to 15% water by weight there are dissolved at 80.degree. and 220.degree. F. about 3.2 pounds and 16.5 pounds trimellitic acid per 100 pounds solvent. Ordinarily aqueous acetic acid of 90 to 98% (10 to 2% water) by weight is used in the oxidation as solvent not only because acetic acid of higher strength is more expensive to recover but also because the presence of 2 to 10% water by weight substantially eliminates oxidation induction. During oxidation of the methyl groups to carboxylic acid groups water is produced as a by-product and is generally retained through the removal of heat of reaction by condensing the acetic acid and water boilup from the liquid phase in the oxidation zone and returning the condensate to the oxidation zone. The aqueous acetic acid solvent in the effluent removed from the oxidation zone can contain about 10 to 25% water (90 to 75% acetic acid) by weight when the 90 to 98% aqueous acetic acid solvent is used in the weight ratios of 5 to 2 parts of pseudocumene. Thus, at usual crystallization temperatures of 60.degree. to 120.degree. F., a substantial amount of trimellitic acid remains in solution.
For example, in Example II of U.S. Pat. No. 3,161,658 there is described the cooling to 100.degree. F. of an oxidation reaction effluent containing for each 500 parts acetic acid solvent, 200 parts trimellitic acid and 50 parts of pseudocumene oxidation intermediates. There was recovered 135 parts crystalline trimellitic acid per 500 parts of acetic acid solvent. Thus, of the originally produced 200 parts trimellitic acid there was left in solution 65 parts or 32.5%. This appears to have been an oxidation of pseudocumene conducted in the presence of acetic acid solvent in the ratio of about 3.5 parts solvent per part of pseudocumene. Higher ratios of solvent to pseudocumene would have caused a greater proportion of the total trimellitic acid to remain in solution at 100.degree. F. For example, at a 5 to 1 solvent ratio 45% of the trimellitic acid produced would have remained in solution at crystallization and filtration temperatures of 100.degree. F.
In manufacture of trimellitic acid, after maximizing recovery of the trimellitic acid from the acetic acid reaction solvent by crystallization and filtration, the mother liquor is stripped to recover the aqueous acetic acid solvent. The residue obtained is a mixture of trimellitic acid, oxygen-containing derivatives of benzene and toluene which are mono-, di- and tricarboxylic acids, aldehydocarboxylic acids, and methylol-substituted benzene, toluene or their carboxylic (benzoic or toluic) acids. The residue also contains components of catalysis. These components of catalysis are Co-Mn-Br or Co-Mn-Br-Zr from the liquid phase oxidation of pseudocumene. While such residue amounts to from 2 to 25 weight percent of the trimellitic acid produced, such residue production annually is substantial in view of the millions of pounds of trimellitic acid produced annually.
Since this residue contains water soluble benzene carboxylic acids and water-soluble forms of the catalyst, landfill disposal of such residue is undersirable because rain and groundwater break out these carboxylic acids and water-soluble forms of the catalyst which are typically cobalt and manganese acetates. Disposal of the organic portion of the residue can be made by processes as disclosed in U.S. Pat. Nos. 4,258,227, 4,266,084 and 4,393,264. The catalyst components in the residue are converted by the processes of these patents to forms in the resultant ash which are difficult and/or expensive to convert to reusable forms for the oxidation of the pseudocumene.
The recovery of the cobalt and manganese and zirconium, if such be present in the catalyst system, would not only reduce catalyst costs but reduce or eliminate any costs associated with disposal of the ash.
It is therefore an object of this invention to recover trimellitic acid from the stripper residue or bottoms and reduce the amount of waste products obtained in the oxidation of pseudocumene to trimellitic acid.
It is an object of the instant invention to increase the yield of trimellitic acid from oxidation of pseudocumene to trimellitic acid by increasing recovery of trimellitic acid.
It is an object of the instant invention to improve the process for preparation of trimellitic acid by oxidation of pseudocumene in presence of a cobalt-manganese or cobalt-manganese-zirconium catalyst with bromine and a solvent comprising acetic acid wherein recovery of trimellitic acid obtained thereby is increased over previously available processes and the catalyst metals are recovered in a form suitable for immediate re-use and therefore, immediate recycle to the oxidation reaction.
It is an object of the instant invention to reduce the amount of waste produced by an oxidation process to prepare trimellitic acid from pseudocumene.
Other objects of the instant invention will be apparent from the discussion which follows.